1. Field of the Invention
The present invention relates to differential amplifiers that amplify voltage differences between input signals.
The present application claims priority on Japanese Patent Application No. 2007-226646 and Japanese Patent Application No. 2008-193864, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Various types of differential amplifiers have been disclosed in various documents such as Patent Document 1.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-261261        
Patent Document 1 discloses a differential amplifier (serving as a bistable multivibrator or a transconductance amplifier) that is designed to suppress nonlinear errors. With reference to FIG. 3 of Patent Document 1, a pair of amplification transistors 1 and 2 are alternately connected with a pair of emitter-follower transistors 11 and 12, wherein emitter resistors 3 and 4 are connected to the emitters of the amplification transistors 1 and 2, while the collectors of the emitter-follower transistors 11 and 12 serve as open collector output terminals.
The output current (Iout1-Iout2) of the differential amplifier is represented by the following equation (in which RE designates the resistance of the emitter resistor), which does not include any terms representing the nonlinearity of transistors; hence, the differential amplifier has a superior linearity over a broad range of the input voltage (Vin1-Vin2).
            Iout      ⁢                          ⁢      1        -          Iout      ⁢                          ⁢      2        =                    Vin        ⁢                                  ⁢        1            -              Vin        ⁢                                  ⁢        2              RE  
With reference to FIG. 5 of Patent Document 1, a differential amplification circuit including amplification transistors 1 and 2 is interconnected with four current mirror circuits 21 to 24 forming a power transmission circuit (or a signal transmission circuit), thus solving drawbacks such as the operational instability of the differential amplifier.
In the amplification transistors 1 and 2 included in the differential amplifier shown in FIG. 3 of Patent Document 1, the emitter-collector voltage VCE is set identical to the base-emitter voltage VBE when no input voltage is applied thereto, whereby the amplification transistors may be easily saturated in operation when an input voltage is applied thereto. That is, the aforementioned differential amplifier suffers from a problem in that the output signal may be easily distorted as the input voltage increases.
The differential amplifier shown in FIG. 5 of Patent Document 1 is designed to solve the aforementioned problem; however, due to the complex constitution of the power transmission circuit and the relatively long propagation path thereof, it is very difficult to achieve high-speed operation.
In differential amplifiers, base-emitter voltages VBE of transistors have temperature dependencies, and transistors may differ from each other in heating conditions due to different power consumptions thereof. For this reason, it may be very difficult to adequately reduce gain errors and nonlinear amplification errors in broad ranges of frequencies. In other words, it is very difficult for differential amplifiers to possess flat gain characteristics in broad ranges of frequencies.